Appendix-B : Flow Regulator Structures

Flow regulators (or diversion structures) are used to control the flow between collection sewers and the interceptors. These regulators allow the conveyance of wastewater to treatment facilities during dry weather conditions. During wet weather conditions the regulators divert flows away from the interceptor and discharge directly into a water course to avoid surcharge and flooding of the combined sewer system. Flow regulator devices include side weirs, leaping weirs, transverse weirs, orifices and relief siphons. Metcalf & Eddy, Inc. (1991) presents a detailed description of these different devices. This particular tutorial will use the transverse weir with orifice type of regulator illustrated in Figure 3.2. In this regulator there is a weir or a small plate placed directly across the sewer perpendicular to the line of flow. Low flows are diverted to the interceptor through an orifice located upstream of the weir. During periods of high flow, the weir is overtopped and some flow is discharged through the overflow outlet, eventually reaching a CSO outfall.

SWMM Representation

A transverse flow regulator can be represented in SWMM by using weir and orifice elements. Because these elements correspond to hydraulic links, additional junction nodes must be added into the model. A schematic representation of three possible definitions of a transverse flow regulator in SWMM is shown in Figure 3.3 Configurations (a) and (b) both contain the weir shown in Figure 3.3, but use different elements to divert to the interceptor: (a) uses a bottom orifice while (b) uses a pipe. Finally, the third configuration (Figure 3.3 (c)) uses neither a weir nor an orifice. Instead, it simply diverts flow by using different inlet offsets for the pipes that convey flows to the interceptor and to the stream. The first pipe has an inlet offset of zero while the pipe linked to the stream has a larger invert elevation.

In this tutorial the combined sewer pipes are made directly connected to the interceptor and the weirs are connected to the storm water conveyance pipes. So, the sewer overflow will move to these pipes when the capacity of the interceptors will be reached. Generally, the choice of a particular configuration to use in a real application will depend on the specific conditions encountered in the field and on how numerically stable the resulting model will be. Some unwanted hydraulic phenomena that can be artificially introduced into the model by these different representations include surcharged weirs, instabilities caused by short pipes and excessive storage associated with large pipes.